Tough tunnelling

Published in the September 2008 Australia’s Mining MonthlyA view of the coal feeder unit inside the TechSpan reclaim tunnel at the Newpac coal mine in New South Wales. A key issue in play is that the moving stockpile means the loads on the tunnel below are unbalanced.The Reinforced Earth Company put its hand up to do the job for not one but two reclaim tunnels. The job, on behalf of Roche Process Engineering, was part of the expanded case mine plan for the Newpac No. 1 colliery in the Hunter Valley region of New South Wales.The mine was operating on a bord and pillar basis with 1.1 million run of mine tonnes produced in financial year 2006. The plan was to take the ROM production to 4Mt per annum in 2007 – thanks in part to newly installed longwall mining equipment. There were also upgraded conveyors, an upgraded Ravensworth coal preparation plant and larger ROM and product coal stockpiles.Two new coal-handling tunnels were needed and this is where RECO came in. One tunnel was to handle the raw coal and the other to handle the clean “product” coal. The raw coal tunnel needed to incorporate three chambers including one for a skid steer loader to turn around.The product coal tunnel needed to incorporate five different chutes to allow coal to be delivered from the stockpile above through a valve onto the conveyor below.RECO opted to supply the two reclaim tunnels with a precast concrete arch design based on the TechSpan system.The intricate arch design had to factor in the various loading cases arising in relation to the height and position of the coal stockpile as well as live load considerations. The design also had to accommodate considerable changes in grade to some tunnel sections.Reinforced earth headwalls and wingwalls were adopted at tunnel entrances.RECO also proposed incorporating voids to the roof of the arch to five locations along the length of the product coal tunnel to accommodate the coal valves. These valves would release coal from the stockpile above onto the conveyer below. To do this RECO designed a unique ring beam system to tie the arches together and support the valves. A number of special arch units were precast to allow for incorporation of the ring beam.This removed the need to construct the more commonly used heavily reinforced in situ box chambers.The precasters were able to turn out six arch units a day, which was a 50% increase on previous jobs of this type. The production requirement was needed to meet the tight construction deadline set on the project.According to RECO, this rate of precasting was exceptional, particularly in view of all the mould changes required due to the high number of “special” units the project required.This was the second such reclaim tunnel supplied to Roche Process Engineering in as many years. Newpac built on the success of a similar tunnel at Foxleigh in Queensland.

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